Which Selene as she was upgraded in some movies.

The strongest version of her versus the strongest version of him.

While, Blade did kill a powerful entity called La Magra and the first Vampire Dracula.

However, Selene should never be underestimated. I think, they are too matched.

Blade killed La Magra using a special chemical he injected him with. Prior to that, La Magra completely outmatched him.

By the end of the series, Selene has displayed feats of speed and strength that far surpassed Blade at his most powerful. The only time Blade would really stand a chance against her is in the first movie, then he might be able to beat her.

After the second movie though, when she gains the Corvinus Strain by drinking from Alexander Corvinus though, it's HIGHLY unlikely he'll be able to beat her.

After she gains her spiritual powers after undergoing the ritual in Blood Wars though, forget it, no way he's beating her.

Both of them got help from different places. So no one is really that powerful on their own.

But we can make comparisons. For example, for speed, Blade at his most power has been able disappear from sight from a human's perspective, while Selene has been able to do it from a David's perspective, and he's fast enough to dodge bullets.

For strength, at his strongest, Blade has shown to be able to lift a ton and press 5 tones. Selene at her strongest was able out rip out Marius' spine. It would take 101-102 tons of force to be able to do that to a human, so to do that to Marius in his pseudo-hybrid form, it would take even more force since he's much more durable than a human.

Now, when it comes to fighting skills, Blade definitely holds the advantage with his superior martial arts. Selene is shown to be a capable fighter too, but not at Blade's level. However, after undergoing the Var Dohr ritual, she gained a level of clairvoyance and other spiritual abilities that would be able to aid her in a hand-to-hand fight against him, plus she she's lived a lot longer and so has a lot more combat experience.

So when it comes to speed and strength, Selene holds the advantage. For durability and recovery, they're on the same level. For fighting skills, Blade has a bit of an advantage, but Selene can make up the difference with her Var Dohr abilities.

Basically, 1st Underworld movie - a close battle that could go either way.

2nd Underworld movie - Selene greatly holds the advantage.

4th/5th Underworld movie - Selene is basically guaranteed a victory.